MR. WHEATSTONE ON THE PHYSIOLOGY OF VISION. 
15 
hind them to be seen. A flowering shrub before a hedge appears to be transferred 
behind it; and a tree standing outside a window may be brought visibly within the 
room in which the observer is standing. 
I have before observed that the transition from the normal to the converse percep- 
tion is often gradual ; I will give one instance of this as an illustration. The object 
was a page of medallions embossed on card-board, and the raised impressions were 
protected from injury by a thick piece of mill-board having apertures in it made to 
correspond to each medallion. The page was placed horizontally, illuminated l>y a 
candle placed beyond it, and looked at through the pseudoscope at an angle of 45 °; 
for the first moment the page appeared as it would have done without the instrument; 
soon after the medallions appeared level with the upper surface, and the shadows on 
the upper parts of the circular apertures were converted into deep depressions as if 
cut out with a tool ; they next, from horizontal, became vertical, each standing erect 
on the horizontal plane, and immediately afterwards the reliefs were all changed into 
hollows ; finally, the page itself stood vertical, but with that change of form which I 
indicated in the case of the rule, the upper edge appearing much shorter than the 
lower edge : the series of changes being now complete, the final form remained con- 
stant as long as the object was regarded. 
In endeavouring to analyse the phenomena of converse perception, it must be 
borne in mind that the transposition of distances has reference only to distances 
from the retinee, not to absolute horizontal distances in space. Thus, if a straight 
ruler be held in the vertical plane perpendicular to the optic base, and also inclined 
45° to the horizon so that its upper end shall be the most distant, when the eyes are 
directed horizontally towards it, the rule will appear exactly in the converse position. 
If the rule be now removed lower down in the same vertical plane, its inclination 
remaining unchanged, so that to look upon it the plane of the optic axes must be 
inclined 45°, it will appear unaltered in position, because its two pictures are parallel 
on the retinse, and the optic axes would require the same convergence to make the 
upper and lower ends coalesce. The rule being removed still lower down, instead 
of its position being apparently reversed, it will appear to have a greater inclination 
on the same side than the object itself has. In the first case the more distant end is 
actually farthest from the eyes ; in the second the near and remote ends are equally 
distant ; and in the third the nearest end is most distant. 
Attention to what I have just stated will explain many anomalous circumstances 
which occur when the eyes are differently directed towards the same object. It may 
also be necessary to remark, that the conversion of distance takes place only within 
those limits in which the optic axes sensibly converge, or the pictures projected on 
the retinae are sensibly dissimilar. Beyond this range there is no mutual transposi- 
tion of the apparent distances of objects with the pseudoscope ; a distant view there- 
fore appears unchanged. 
Some very paradoxical results are obtained when objects in motion are viewed 
